The present invention relates in general to motor control and in particular to a new and useful circuit for controlling the speed of a motor utilizing two series connected electronic switches which are controlled in phase opposition by power line frequency through storage units such as capacitors.
Motors having a constant magnetic field are usually equipped with a tacho-generator, for determining the actual speed. This generator produces a frequency which is proportional to the motor speed and is then compared with the preset desired speed of the motor, in order to adjust the speed through a controller stage. The costs of such a tacho-generator, particularly for fractional hp motors, are in no economically reasonable proportion to the costs of the motor.
A switching arrangement is known for controlling the speed of motors (German AS No. 26 08 613) in which the armature emf is periodically measured and used for controlling the duration of the next cut-in period.
In this arrangement, the emf which is proportional to the motor speed is isolated in a very complicated and expensive way, so that the manufacture is as costly as the abovementioned arrangements with a tacho-generator.